The present invention relates to the machining of slots by use of a rotational cutting tool. More particularly, the invention provides an end mill, which can be used for different machining purposes by changing the distance between a thin workpiece and the end mill shank. This change is accomplished either by moving the workpiece or the cutter.
A common requirement is to machine a slot, straight or contoured, in a thin workpiece. “Thin” means that the machined item is usually no thicker than the tool diameter.
If the required slot reaches an outer edge of the workpiece, or the operation to be carried out comprises machining the outer edge, then the first operation is rough machining, leaving a little material for finishing, and the following operation is finish machining to the required size.
End-mills configured for rough machining are generally unsuitable for finish machining. During rough machining much material is removed, and the removal of chips from the cutting area requires adequate space between cutting teeth. Such space can only be provided when the number of teeth is low, typically 2–4 teeth depending on cutter diameter, and the core diameter of the tool is small, although still large enough to prevent tool breakage. The smaller core diameter may lead to minor loss of accuracy due to tool bending under pressure, but this is of little moment for roughing operations.
Conditions for finish machining are quite different. A larger number of teeth will produce a smoother finish, and as only small quantities of material are removed, little space is needed between teeth for chip clearance. Accuracy of cut is essential for finishing operations, and therefor a large core diameter is desirable to provide the necessary tool rigidity to produce smooth and accurate surfaces. In choosing an appropriate core diameter, there is always a need to balance the requirements of chip clearance and coolant access on the one hand, which call for a small core diameter, and the demand for tool rigidity and break resistance which require a large core diameter.
According to the recommendation of. The Cincinnati Milling Machine Co., no more than two teeth at a time should be engaged in a cut. This rule corresponds well with our previous remarks. During roughing fewer teeth are preferred as the cut is deep; during finishing more teeth are desirable as the cut is shallow.
There are further differences between roughing and finishing tools. The ideal tooth helix angle of an end-mill is partly dependent on the number of teeth. A roughing cutter having few teeth (2–3) requires a large helix angle (about 25°) to reduce vibration. However a finishing cutter having more closely spaced teeth will produce a better finish with a more moderate helix angle; 10° is usual.
For these reasons the tool should be changed after completing the roughing cut and before starting the finishing operation and changed again for the roughing operation on the next workpiece. This is tiresome when done by hand, and still time consuming even when carried out by an automatic machine. The alternative is to complete all roughing cuts on all the workpieces, and then carry out the finish machining on the whole batch. This method however involves clamping and unclamping each workpiece, and aside from the time needed for this, there is greater risk of inaccuracy as a result of the workpieces not being clamped with the same force each time, or as a result of a chip lodged between the workpiece and a locating surface of the fixture or machine vice or table.
If the required slot does not reach an outer edge, then the first operation is drilling a hole to allow entry for the end-mill. Many end-mills are provided with cutting teeth at the end face of the tool and it is possible to use such an end-mill for drilling. However such operation requires high pressure, machining time is extended and the hole produced could be oversize or even off its intended position. A drill can be used to produce the hole, but of course this involves more changeover time. After the hole is drilled, the slot is rough machined and then finish machined as described above.
In U.S. Pat. No. 4,411,563 Moon discloses an end mill provided more thin one set of cutting teeth for the purpose of forming a plurality of steps at the closed extremity of a bored hole. Although the proposed cutting tool will carry out several machining tasks, there is no separate provision for rough and finish machining.
The same remark applies to the double diameter boring tool disclosed by Ashbaugh et al. in U.S. Pat. No. 4,793,745. The tool produces a bore with a stepped diameter at its closed end.